1. Field of the Invention
This invention relates to a method for changing the inert gaseous content of a volume containing hydrocarbon vapors to air without producing an explosive mixture. More specifically, the invention relates to changing the inert gaseous content of a discharged crude-oil tank containing inert gas including hydrocarbon vapors to air without producing an explosive mixture in the presence of air.
2. Description of the Prior Art
Ocean oil-transportation companies are very interested in finding procedures and systems which will reduce the risks of fires or explosions occurring aboard their vessels, particularly those related to fires or explosions in the cargo tanks. As a result, a number of inerting systems have been installed on VLCC's (very large crude carriers) to displace the non-liquid content of these cargo tanks as a safeguard. Although an inerted cargo tank cannot support a fire or an explosion, a difficulty may arise when an inerted tank is being ventilated with air, that is, gas-freed in order to replace the inert gas with outside air. This gas-freeing of a tank allows men to freely enter the tank to do repairs without being poisoned or asphyxiated, as well as eliminating the potentially explosive hydrocarbon vapors. It is noted that this same problem occurs in petroleum refineries and other places where hydrocarbon vapors and air may mix.
Inerted tanks contain too little oxygen to support combustion; though they may contain, either throughout their volume or in isolated pockets, high concentrations of hydrocarbon vapors. When such a tank is ventilated with air, the tank atmosphere or inert gaseous content including hydrocarbon vapors may reach a stage of dilution where the supplied air has provided enough oxygen to mix with the hydrocarbon vapors and produce a combustible mixture that will explode when exposed to a source of ignition.
The current practice to eliminate the possibility of a combustible mixture forming is to purge the tank with a predicted flow volume of inert gas for a prescribed time, for example two hours, which has been empirically demonstrated to produce relatively hydrocarbon vapor-free volumes. Yet, there are three drawbacks to this method: one is that the particular system may be different from its state when a previously "safe" purge time was determined since hydrocarbon vapors may be formed from the residue in the tank during purging; second, the operation wastes both inert gas and time, since generally purging is continued longer than needed; third, a situation may arise when the volume of inert gas that was delivered was less than predicted.
Another way to test inerted tanks of low oxygen content is to mix a sample from the tank with air by mechanical flow proportioning devices, such as rotameters and pumps, before testing. Though these devices result in reliable data, they are not portable.
Another device, called a "flow proportioning valve," is available that connects to portable gas analyzers, detectors or indicators consisting of interchangeable orifices of different sizes which fix the percentage of air mixed with a sample. Since presently available detectors need oxygen of 11 to 21% of the volume to function, they cannot operate without such a valve when they are used to measure a sample from an inerted tank which contains an oxygen volume of less than 11%. The flow proportioning valve, however, is sensitive to the resistance of the sample feed line; thus, air proportion varies as the feed line and orifices accumulate foreign matter or the feed line length increases. For example, for a given orifice setting, the percentage of air dilution can vary from 24% with 1/8 inch-diameter tube, 2 feet long, to 51% for a similar tube 4.5 foot long, to 69% for a tube 15 feet long. Further distortion of readings is due to possible leaks in the feed line and the positive pressure that must be maintained in the inert gaseous volume to prevent the entrance of outside air in the volume.
The present invention can eliminate or substantially reduce the error that may be introduced by currently available system.